Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal infectious neurodegenerative disorders in man and animals (Prusiner 1982, 1998). Examples are Creutzfeldt-Jakob disease (CJD), variant CJD (vCJD), variably protease-sensitive prionopathy (VPSPr), Gerstmann-Sträussler-Scheinker syndrome (GSS), fatal familial insomnia (FFI), and kuru in humans; bovine spongiform encephalopathy (BSE) or mad cow disease in cattle, scrapie in sheep and goat, and chronic wasting disease (CWD) in cervids. Prions use template-directed refolding of the normal cellular prion protein (PrPc) into the pathologic isoform PrPSc for propagation (Prusiner 1982, 1998). This epigenetic process does not involve the coding of nucleic acids in the infectious agent and is solely based on change in protein conformation.
In humans, prion disease can be initiated by a spontaneous event, with genetic linkage passing from generation to generation within families, or acquired by infection. Examples of routes for infectious prion transmission include blood transfusions, dura mater grafts, and contaminated human growth hormone or contaminated medical instruments (iatrogenic prion diseases). Although rare, every year about 8,000 people die of sporadic and genetic prion diseases worldwide, and patients with genetic predisposition to prion infection can be diagnosed long before the onset of clinical disease presentation. As a result of BSE, there is evidence that between 1:10,000 and 1:20,000 in the general population of U.K. are infected with vCJD prions and are incubating the disease. So far, there is no established therapy or prophylaxis for human prion diseases. The major limitations of experimental anti-prion drugs include severe side effects observed in animal models and inability of the investigational drug to cross the blood brain barrier (BBB).
Cell culture models persistently infected with prions are typically used to screen potential anti-prion compounds for activity (Nunziante et al., 2003; Gilch et al., 2008; Krammer et al., 2009). In these models, treated and control cells are analyzed for the amount of PrPSc, which serves as a surrogate marker for prion infectivity. In this physiological system, the cellular and molecular requirements for conversion and cellular turnover of prions are considered, whereas most in vitro assays only test for interference in the physical interaction of PrPc and PrPSc (Nunziante et al., 2003; Gilch et al., 2008; Krammer et al., 2009). These requirements include, for example, the proper subcellular localization and trafficking of PrPc and PrPSc as well as the degradation kinetics of PrPSc. Validation of potential drug targets can be performed in prion-infected animal models.
A promising experimental anti-prion strategy is the induction of autophagy. Autophagy is a basic cellular program for degradation and recycling of cytosolic proteins, protein aggregates, and organelles. Published data shows that autophagy is a potent modifier of the cellular clearance of prions and that drug induced autophagy shifts the delicate equilibrium between propagation and clearance of prions towards the latter (Ertmer et al., 2004, 2007; Aguib et al., 2009; Heiseke et al., 2009, 2010). There is proof-of-concept evidence that drug-induced activation of autophagy can delay or diminish prion diseases in animal models (Aguib et al., 2009; Heiseke et al., 2009).
AR-12 (a.k.a. OSU-03012) has been previously shown to exhibit anti-tumor, antiviral, anti-fungal and anti-bacterial activity. It is thought that AR-12 induces autophagy of cells harboring intracellular microbes. However, the anti-prion activity of AR-12 has not been previously shown.